The present invention relates to a floppy disk drive (hereinafter referred to as FDD) window signal gate circuit used in an FDD interface mechanism in which the data transfer rate of an FDD is unknown.
In a conventional 5.25" FDD, use is made of a recording medium having either double-sided, double-density, double tracks (2DD type) or double-sided, double-density tracks (2D type). The recording medium is driven at 300 rpm by a spindle motor. The data transfer rate in the read operation is 250 kbps.
A recording medium having double-sided, high-density, double tracks (2HD type) has also been developed. When this medium is driven, the rotational frequency of the spindle motor for an FDD is 360 rpm, and the data transfer rate in the read operation is 500 kbps.
A disk drive for 2HD type medium can drive a recording medium of 2DD or 2D type. Since the rotational frequency of the spindle motor used in this disk drive is 360 rpm, the data transfer rate is 300 kbps.
In some 2HD type disk drives, the rotational frequency of the spindle motor is switched between 360 and 300 rpm. However, an extra wait time must be required for switching the rotational frequency of the spindle motor.
It is desired that an FDD interface circuit be compatible with three data transfer rates of 250 kbps, 300 kbps, and 500 kbps. A conventional FDD interface circuit consists of voltage frequency oscillator (VFO) 2, floppy disk controller (FDC) 3, and switching circuit 4, as shown in FIG. 1. Data 10 read from floppy disk drive unit (FDD) 1 is supplied to VFO 2. VFO 2 creates read data 11 and window signal 12 from data 10, and supplies them to FDC 3. FDC 3 receives read data 11 and window signal 12. Switching circuit 4 supplies a signal (250 kbps, 300 kbps, or 500 kbps) for switching the data transfer rate to VFO 2 and FDC 3.
Host system 5 reads data from FDD 1. Host system 5 and FDC 3 are connected by bus 14, and host system 5 and switching circuit 4 are connected by bus 15. Even if the data transfer rate of data 10 slightly fluctuates by VFO 2, FDC 3 can correctly read ID information and data. Therefore, the data transfer rate has some allowance in all of the 250 kbps, 300 kbps, and 500 kbps modes. This allowance is called a capture range. In the 250 kbps and 300 kbps modes, the center frequencies are close to each other. Therefore, if the capture range of VFO 2 is wide, the transfer rates overlap as shown in FIG. 2. When this occurs, in the 300 kbps mode, data written in the 250 kbps mode is read as indicated by the broken lines in FIG. 2. Conversely, in the 250 kbps mode, data written in the 300 kbps mode is read. Therefore, host system 5 cannot correctly identify the mode.